The U.S. Navy has built and operated exclusively nuclear-powered submarines for the past several decades. Nuclear power in many ways embodies the American spirit: bigger, stronger, faster, and more powerful than anyone else. However, it is time to challenge the need for those characteristics in submarines. Non-nuclear air-independent propulsion (AIP) submarines offer advantages over nuclear submarines, as seen in submarine development in countries such as Germany and Sweden. Conventional diesel-electric engines are a popular means of propulsion for non-nuclear boats, and there are several AIP options, but hydrogen-powered fuel cells offer more than any other option.
Fuel cells are not new. They have undergone significant technological improvements from when they were first considered for submarine propulsion by Germany in the 1950s. The principle of producing power is straightforward; hydrogen and oxygen gas react to produce water and an electrical current. It is the reverse process of electrolysis, where a current is sent through liquid water to split the bonds between the oxygen and hydrogen atoms. Through engineering optimization, enough electrical power can be harnessed from this reaction to power a variety of loads. Current uses include cars, buses, remote cell phone towers, and forklifts. The German Navy already has a hydrogen fuel cell–powered submarine class, the Type 212, first launched in 2005, and variants it sells abroad to countries such as Italy and Singapore.
Fuel Cells Are Stealthy
The Gotland-class submarine, a Swedish boat, is the most prominent example of the extreme stealth of non-nuclear AIP submarines. During a joint wargaming exercise in 2005, it tactically sank the USS Ronald Reagan (CVN-76) several times. It was virtually undetectable by all available antisubmarine efforts.1
While powered by a Stirling engine, the concept and application of the Gotland-class AIP system are the same as for others. Stirling engines and other forms of non-nuclear AIP, while quieter than nuclear, are louder and less efficient than fuel cells. There are no mechanical parts in the main fuel cell system such as in combustion driven engines. Fuel cells offer the lowest noise levels because almost no sound is produced by an electro-chemical reaction. The only components in the engine room that could contribute to the sound signature are the compressors and pumps for fuel, water, and cooling.
Yet, cooling requirements for fuel cells are much lower than combustion and nuclear because of the low operating temperature of 100°C for proton-exchange membrane fuel cells. Conversely, nuclear-powered submarines need extensive cooling and vibrational dampening because of high operating temperatures, requiring several large coolant pumps and bulky, complicated mechanical systems such as steam turbines and reduction gears.
In addition to the fuel cells, there are advanced lithium-ion batteries on board AIP vessels that can power the electric motor at higher speeds with no loss of acoustic fidelity. Without the nuclear reactor, there also is a smaller infrared heat signature and no radiological trace. There is a significant stealth advantage to fuel cells that lowers the detectable range of the vessel.
Fuel Cells are Efficient
Fuel cell AIP submarines do not have the nominally infinite endurance of nuclear-powered submarines; however, they can remain underwater for much longer than alternative AIP options such as closed-cycle diesel generator, Stirling, and MESMA (a French steam turbine).2 Fuel cells are significantly more efficient than diesel engines, thus requiring less oxygen fuel per kWh of energy produced. Diesel-electric boats have a max underwater time of a couple of days because of battery limits. Fuel cell AIPs can last weeks underwater and have a range of up to 2,000 nautical miles. Further, by forward deploying these vessels in ports that are close to their respective operating areas, the ratio of time on station to transit and refueling time is increased.
Fuel Cells Increase Mobility
Because of their more limited endurance, non-nuclear AIPs perform most effectively close to shore in the littorals, placing an emphasis on mobility. The smaller size of these submarines decreases their turning radius; the Type 212 is about half the length of a Virginia-class submarine, allowing for faster turns to avoid threats, pursue targets, or maneuver around areas of high sensitivity, including shallow water. With an electric drive propulsion system, combinations of vectoring propulsors or thrusters could add improvements to maneuverability.
Another aspect of increased mobility is the ability of the submarine to “bottom,” or sit at the ocean floor, with only critical systems online to preserve energy and extend the operational time while using passive sonar to detect targets. Since fuel cells operate with greater efficiency at lower loads, bottoming could extend the endurance of a particular mission.
Fuel Cells are Tactical
Non-nuclear AIP submarines do not fulfill the same offensive mission as nuclear attack submarines in the open ocean, but their firepower capability can meet defensive and offensive demands in the littorals. These submarines can rest on the bottom of shallow waters and act as an undetectable defensive force on the coast of the United States and abroad. In addition, because they are quieter and highly maneuverable, they have an increased ability to engage targets with torpedoes. Even the insertion of special operations forces in sensitive areas is more manageable.
Because of the increased capabilities of these versatile submarines, a training facility similar to TopGun could be established to train sailors to maximize the operational ability. A TopGun fleet could possibly act as an “aggressor” or “red” team in larger training operations with joint forces. This would allow expensive nuclear warships to stay operational. It also would enable crew members to learn the tactics of enemies, especially Russia and China, which already have similar AIP designs, such as the Lada and Yuan classes.
Fuel Cells are Affordable
These smaller, non-nuclear submarines are much more affordable. The cost of one Virginia-class boat is about $2.7 billion, while the cost of one German Type 212 is about $500 million. The Chinese Yuan-class diesel-electric and Russian Lada-class hydrogen fuel-cell submarines are new programs of comparable size and mission. At only about 200 feet in length, displacing around 2,000 long tons submerged, and costing from $100 million to $500 million, the relative lethality per dollar for such vessels is unprecedented. Fortunately, the U.S. Navy worked with the Gotland to learn non-nuclear AIP tactics to know how to fight them in the future.
Non-nuclear submarines also are less expensive to maintain and man. Fuel cell systems are modular and can be replaced quickly and easily, and these submarines are manned by crews of 25–35 people. Compared to a Virginia-class submarine, that is about a quarter of the crew and billions of dollars in personnel costs saved over the life cycle of the ship.
The submarine force is a foundation of U.S. command of the seas. As the world changes, new technology is needed to combat new threats. The U.S. Navy may no longer be able to maintain a competitive edge by being the biggest and fastest, but rather by being the quietest and most deadly.
1. Sebastian Roblin, “Sweden’s Super Stealth Submarines Are So Lethal They ‘Sank’ a U.S. Aircraft Carrier,” The National Interest, 13 November 2016.
2. J. B. Lakeman and D. B. Browning, The Role of Fuel Cells in the Supply of Silent Power for Operations in Littoral Waters, QuinetiQ Centre for Marine Technology, Halsar (April 2004).